The light-emitting systems in organic electronics may fundamentally be classified in two different groups. On the one hand, the longer known and commercially available systems, which are capable, utilizing fluorescence properties of organic or organic/inorganic complexes, of converting electrical energy into light. On the other hand, systems, the conversion properties of which are based on electronic transitions, which can be associated with phosphorescence. The latter are at least theoretically capable, in consideration of the applicable quantum statistics, of reaching an internal quantum efficiency of 100%. This is in contrast to solely fluorescent emitters, which can have a maximum internal quantum yield of only 25% because of the quantum statistics.
A previously followed approach was to use metal complexes having metals of the sixth period as the emission or absorption centers. Phosphorescence also does occur in conjunction with the elements of the fourth and fifth periods of the periodic system, but the complexes of the metals of the sixth period have proven themselves. Depending on the location of the elements in this period, the origin of the phosphorescence is weighted differently within the orbital structure of the complexes in this case.
In the lanthanides, both the HOMO (highest occupied molecular orbital) and also the LUMO (lowest unoccupied molecular orbital) are predominantly metal-centered, i.e., the component of the ligand orbitals is relatively weakly pronounced. As a result, the emission wavelength (color) of the emitters is established almost exclusively by the band structure of the metal (examples europium=red, terbium=green). Because of the strong shielding of the f electrons of these metals, ligands coupled to the metal are capable of splitting the energies of the fn configuration of the metals only around approximately 100 cm−1, so that the spectroscopy due to their ligand field significantly differentiates the d ions from that of the f ions. In ions of the lanthanides, the color results from transitions of f into unoccupied s, p, and d orbitals.
If one travels along the period to the elements osmium, iridium, platinum, and gold, ligand fields thus split the metal orbitals by a factor of 10-100 times more than in the case of the lanthanides. Therefore, by variation of the ligands, practically the entire visible wavelength spectrum may be represented using these elements. Due to the strong coupling of the orbital angular momentum of the metal atom to the spin angular momentum of the electrons, phosphorescence is obtained in the emitters. The HOMO is usually metal-centered in this case, while the LUMO is usually ligand-centered. The radiant transitions are therefore referred to as metal-ligand charge transfer transitions (MLCT).
Both OLEDs (organic light-emitting diodes) and also OLEECs (organic light-emitting electrochemical cells) presently use almost exclusively iridium complexes as phosphorescent emitters. In the case of OLEDs, the emitter complexes are uncharged; in the case of OLEECs, ionic, i.e., charged emitter complexes are used. The use of iridium in these components has a severe disadvantage, however. The yearly production of iridium is well below 10 tons (3 tons in 2000). This has the result that the material costs provide a significant contribution to the production costs of organic electrical components. In addition, iridium emitters are not capable of efficiently imaging the entire spectrum of visible light. Thus, stable blue iridium emitters are rather rare, for example, which opposes a flexible use of these materials in OLED or OLEEC applications.
In more recent literature, in contrast, there are some approaches which propose “triplet harvesting” also using emitters which are not based on iridium. Thus, for example, Omary et al. in “Enhancement of the Phosphorescence of Organic Luminophores upon Interaction with a Mercury Trifunctional Lewis Acid” (Mohammad A. Omary, Refaie M. Kassab, Mason R. Haneline, O. Elbjeirami, and Francois P. Gabbai, Inorg. Chem. 2003, 42, 2176-2178) refer to the possibility of achieving sufficient phosphorescence of solely organic emitters by way of the use of mercury. Due to the heavy atom effect of mercury in a matrix made of organic ligands, a singlet-triplet/triplet-singlet transition of the excited electrons of the organic matrix is enabled in quantum mechanics (ISC, inter-system crossing), which results in a significant reduction of the lifetime of the excited electronic (triplet) states and avoids undesired saturation of the occupation of said states. The cause of this mechanism is the spin-orbit coupling of the mercury heavy atom to the excited electrons of the organic matrix. In contrast, it is disadvantageous that the use of mercury is problematic as a result of toxic and environmental-political aspects.
WO 2012/016074 A1, in contrast, describes a thin layer comprising a compound of the formula
wherein Ar1 and Ar2 are each independently a C3-30 aromatic ring; R1 and R2 are a substituent; a and b are each independently an integer from 0 to 12, wherein, if a is 2 or more, each residue R1 is optionally different from one another, and two residues R1 are optionally bonded to one another to form a ring structure, and, if b is 2 or more, each residue R2 is optionally different from one another and two residues R2 are optionally bonded to one another to form a ring structure; A1 is any type of a direct bond, —O—, —S—, —S(═O)—, —S(═O)2—, —PR3—, —NR4—, and —C(R5)2—; R3 is a hydrogen atom or a substituent; R4 is a hydrogen atom or a substituent; R5 is a hydrogen atom or a substituent and two residues R5 are optionally different from one another; E1 is a monovalent residue having 50 or fewer carbon atoms; L1 is a ligand having 50 or fewer carbon atoms; c is an integer from 0 to 3, wherein, if c is 2 or more, each residue L1 is optionally different from one another; and each combination of a combination of E1 and Ar1 and a combination of E1 and Ar2 optionally forms a bond; and, if c is 1 to 3, each combination of a combination of L1 and E1, a combination of L1 and Ar1, a combination of L1 and Ar2, and a combination of L1 and L1 optionally forms a bond. In contrast, it is disadvantageous that the described compounds only have an inadequate quantum yield and are not sufficiently stable in solution, so that they decompose.
DE 103 60 681 A1 discloses main group metal-diketonato complexes according to the following formula
as phosphorescent emitter molecules in organic light-emitting diodes (OLEDs), wherein M can be Tl(I), Pb(II), and Bi(III). Furthermore, the use of these main group metal-diketonato complexes as light-emitting layers in OLEDs, light-emitting layers containing at least one main group metal-diketonato complex, an OLED containing this light-emitting layer, and devices which contain an OLED according to the invention are disclosed. In contrast, it was possible to show in experiments that the above-mentioned compounds, which are synthesized under strict water exclusion, do not display emission based on phosphorescence after electronic excitation. It is highly probable that the described phosphorescent emissions originate from oxo clusters which are not definable in greater detail, and which have formed in an uncontrolled manner, for example, by hydrolysis in the scope of the production. This special solution has the disadvantage that the n-system of these acetyl acetonate ligands, in particular the described fully fluorinated variants, is less pronounced and only permits low phosphorescence yields as the sole phosphorescent emitter.